Sex Machines

Sex machines solve one specific problem: sustaining a pace and a depth for longer than a person comfortably can.
That is the entire proposition, and it is either exactly what you are looking for or beside the point. These are the largest investment in the shop, and the two specifications that matter are stroke length and whether the mount accepts standard attachments.
If you are unsure, you probably want a thrusting toy rather than a machine — a similar motion, at a fraction of the price and the storage.